When OTFS was introduced, it was presented as an overlay for OFDM systems (see [1]}). The OFDM implementation of OTFS uses the so-called symplectic finite Fourier transform (SFFT) and the inverse SFFT (ISFFT). This transform pair allows mapping symbols defined in the DD domain to the TF domain and vice versa. The overlay for OFDM was initially intended to allow for the reuse of existing hardware [1]}, but later became the standard formulation for research on OTFS. For instance, the overlay for OFDM was used in [3]}, [4]}, [5]} to study the input-output relation for TF dispersive channels in the DD domain.
